Modelling borehole temperatures in Southern Norway – insights into permafrost dynamics during the 20th and 21st century

Abstract

Abstract. This study aims at quantifying the thermal response of mountain permafrost in southern Norway to changes in climate since 1860 and until 2100. A transient one-dimensional heat flow model was used to simulate ground temperatures and associated active layer thicknesses for nine borehole locations, which are located at different elevations and in substrates with different thermal properties. The model was forced by reconstructed air temperatures starting from 1860, which approximately coincides with the end of the Little Ice Age in the region. The impact of climate warming on mountain permafrost to 2100 is assessed by using downscaled air temperatures from a multi-model ensemble for the A1B scenario. Borehole records over three consecutive years of ground temperatures, air temperatures and snow cover data served for model calibration and validation. With an increase of air temperature of ∼1.5 • C over 1860-2010 and an additional warming of ∼2.8 • C until 2100, we simulate the evolution of ground temperatures for each borehole location. In 1860 the lower limit of permafrost was estimated to be ca. 200 m lower than observed today. According to the model, since the approximate end of the Little Ice Age, the active-layer thickness has increased by 0.5-5 m and >10 m for the sites Juvvasshøe and Tron, respectively. The most pronounced increases in active layer thickness were modelled for the last two decades since 1990 with increase rates of +2 cm yr −1 to +87 cm yr −1 (20-430 %). According to the A1B climate scenario, degradation of mountain permafrost is suggested to occur throughout the 21st century at most of the sites below ca. 1800 m a.s.l. At the highest locations at 1900 m a.s.l., permafrost degradation is likely to occur with a probability of 55-75 % by 2100. This implies that mountain permafrost in southern Norway is likely to be confined to the highest peaks in the western part of the country. Background and objectivesPermafrost in general and mountain permafrost in particular experiences increasing interest due to its sensitivity to climate variation and importance for geomorphologic and geotechnical processes (Harris et al., 2009), such as slope stability and natural hazards (Gude and Barsch, 2005;Huggel et al., 2010; Gruber et al., 2004a;Fischer et al., 2006;Haeberli, 1992). There is a need to address the response of ground temperatures (GT) to climate forcing, especially the modulation of the response of GTs to the effect of snow cover and different types of surficial material and bedrock.This study aims at the quantification of subsurface warming and changes in active layer thickness (ALT) over a ca. 250 yr period from the approximate end of the Little Ice Age (LIA) in the mid 19th century to 2100 at three highmountain sites in Southern Norway. Significant warming occurred during that period and is expected to continue. In relation to these changes, we intend to identify the possible zonations of former, present and future permafrost. Finally, we aim to characterise these responses for different...

The relative age of mountain permafrost — estimation of Holocene permafrost limits in Norway

“…In southern Norway three altitudinal transects of boreholes are available from Juvflye, Tronfjell and Jetta. These boreholes are between 10 and 130 m deep, and are drilled in bedrock, blockfields and thick cover of diamicton (Isaksen et al, 2001;Farbrot et al, 2011;Hipp et al, 2012). In northern Norway most boreholes are drilled in bedrock, except for Iskoras in Finnmark, where one borehole site has a moderate diamicton cover.…”

“…3). More details about the calibration and validation procedures are published by Hipp et al (2012) and Etzelmüller et al (2011).…”

“…The subsurface temperature distribution was simulated by numerically solving the transient 1D heat equation for non-uniform coefficients (see also Farbrot et al, 2007;Etzelmüller et al, 2011;Hipp et al, 2012): (Williams and Smith, 1989). As boundary conditions, we prescribe time series of ground surface temperature (GST) and the geothermal heat flux (Q geo ) at the top and bottom ends of the domain, respectively.…”

Future permafrost conditions along environmental gradients in Zackenberg, Greenland

“…This rough treatment of summer surface temperatures (which has been applied in previous modeling studies, e.g., Hipp et al, 2012) is focused on seasonal averages and can not reproduce surface temperatures on shorter timescales, e.g., the daily cycle. As a result, a comparison of temperatures in upper soil layers is less meaningful than for deeper layers, which are only influenced by seasonal or even multi-annual average temperatures.…”

Freeze/thaw processes in complex permafrost landscapes of northern Siberia simulated using the TEM ecosystem model: impact of thermokarst ponds and lakes

“…Since small changes in soil temperature within the freeze/thaw range will result in a large change in apparent heat capacity, an iterative procedure is required to ensure that only small temperature changes occur during each time interval (Nicolsky et al, 2007). This method is commonly applied in permafrost models (Goodrich, 1978;Nicolsky et al, 2007;Dall'Amico et al, 2011;Hipp et al, 2012; and has also recently been applied in a land surface model (Ringeval et al, 2012). Although the method is more physically realistic it requires greater computing resources, which may lead to limitations in the spatial resolution, the length of time that can be modeled, and the number of simulated land surface classes, etc.…”